The OS Group of Technische Universitaet Dresden (TUD:OS) has released a live demo CD of their custom operating system project. TUD:OS is a microkernel-based operating system targeted at secure and real-time systems. Some highlights of the demo CD include a new approach for securing graphical user interfaces called Nitpicker, multiple L4Linux kernels running at the same time on top of a custom L4 microkernel, a survey on the reuse of device drivers on the TUD:OS platform, native Qt-applications, the DOpE windowing system, games, and much more. More information is available at the demo CD website. And yes, boys and girls, there are screenshots, too.

I don't know Minix3 in particular, but in general Minix is a traditional microkernel architecture. L4 is usually refered to as a "nanokernel"; it is an order of magnitude smaller than a typical microkernel, and has only the minimal amount of priviledged functionality needed. I think the original l4 implementation was 13kbytes.

All higher-level functionality is implemented by user-space servers. It is sort of the same philosophy as a microkernel, but taken to the purist extreme to keep implementation clean, optimized, efficient, and secure.

It is notable that one user-space server that has been implemented is the Linux kernel, and another was the Hurd/Mach microkernel, both of which were "hung" onto the L4 in such a way that multiple instances of them can run simultaneously, with approximately a 5% performance penalty.